1. Field of Invention
The present invention relates to a thermal printer that has a platen roller to which a thermal head is pressed and which is removably installed to the main frame of the printer, and relates more particularly to a thermal printer that has a mechanism for precisely positioning the removable platen roller to the main frame.
2. Description of Related Art
The thermal head in a line thermal printer has a printing surface that covers the full width of the printable line, and is urged from behind against the platen roller by a head pressure spring. In order to replace the thermal paper or other printing paper or clean the print head, the thermal head must be separated from the platen roller and held with a gap between the thermal head and the platen roller so that the new paper can be passed between the thermal head and the platen roller or the thermal head can be cleaned, see for example, Japanese Unexamined Patent Appl. Pub. JP-A-2000-318260 teaches a removable platen roller that can be completely removed from the main frame of the thermal printer so that these tasks can be completed easily.
With the thermal printer taught in JP-A-2000-318260, the bearings attached to the opposite ends of the platen roller shaft can be inserted into and removed from slots formed in the main frame. The thermal head and a lock arm are also supported to pivot in opposite directions on a common support shaft. A head pressure spring is disposed between the thermal head and the lock arm, and the bearings that are on the ends of the platen roller shaft and inserted to these slots are pressed and held by the thermal head against the inside end surface of the slots and the curved surface of the lock arm by the force of the head pressure spring.
The position of the removable platen roller in this thermal printer is determined by the platen roller being held between the point of contact between the bearing and the curved end surface of the lock arm and the point of contact between the thermal head and the platen roller, and being fit into the slots in the main frame. The lock arm and the thermal head are attached to the main frame so that the lock arm and the thermal head can pivot. As a result, when the positions of the lock arm and the thermal head change, the position of the platen roller to the main frame also changes. This can prevent precisely positioning the platen roller to the main frame.
A driven gear to which torque is transferred through a speed reducing gear train from a paper feed motor disposed to the main frame is coaxially disposed to the end part of the platen roller shaft. The platen roller must also be precisely positioned to the main frame in order for the driven gear of the platen roller to consistently mesh accurately with the drive gear at the last stage of the speed reducing gear train that is disposed to the main frame.
Because the related art cannot precisely position the removable platen roller to the main frame in certain cases, the driven gear may not mesh properly with the drive gear when the platen roller is installed to the main frame.